Ferrite broadband semi-notch antenna

ABSTRACT

A ferrite-loaded semi-notch antenna, especially suitable for flush mounting in the leading or trailing edge of an aerodynamic surface of an aircraft, and which is capable of functioning in both transmit and receive modes. Combines a short, folded dipole, with an active notch. The electrical length of the antenna is increased over conventional dipole and/or notch antennas by means of end loading in the form of ferrite rings encircling the ends of the dipole. A part of the energy radiates from the conducting surface surrounding the notch, and the remaining (major) portion radiates from the elements located within the notch.

United States Patent [1 1 Stang Nov. 6, 1973 [54] FERRHTE BROADBANDSEMI-NOTCH 2,636,987 4/1953 Dome 343/708 ANTENNA 1 Primary Examiner-EliLieberman [75] Inventor. Paul F. Stang, Saugus, Calif. y g C. Sullivanet a1. [73] Assignee: Lockheed Aircraft Corporation,

Burbank, Calif. 57 ABSTRACT [22] Filed: July 3, 1972 A ferrite-loadedsemi-notch antenna, especially suitable for flush mounting in theleading or trailing edge [21] Appl 268850 of an aerodynamic surface ofan aircraft, and which is capable of functioning in both transmit andreceive [52] US. Cl 343/708, 343/747, 343/787, modes. Combines a short,folded dipole, with an active 343/789 notch. The electrical length ofthe antenna is increased [51] Int. Cl. H01q 1/28 over conventionaldipole an r notch n a by [58] Field of Search 343/705, 708, 789, meansof end loading in the form of ferrite rings encir- 343/747, 787 clingthe ends of the dipole. A part of the energy radiates from theconducting surface surrounding the [56] References Cited notch, and theremaining (major) portion radiates from the elements located within thenotch.

14 Claims, 2 Drawing Figures 1 FERRITE BROADBAND SEMI-NOTCH ANTENNABACKGROUND OF THE INVENTION I-Ieretofore it has been common .to employferrites in conjunction with electric antennas for increasing theefficiency and decreasing the size of the antenna structure. Two suchantennas are disclosed in US. Pat. Nos. 2,748,386, entitled AntennaSystems, and 3,295,137, entitled Shortened Folded Monopole withRadiation Efficiency Increased by Ferrite Loading. However, it isimpossible to flush mount such antennas on an aircraft so as to obviatedrag penalties. Toovercome the drag problem it has been proposed,heretofore, to employ slot or notch antennas since their flushmountedconfiguration is aerodynamically desirable. One such prior antenna isdisclosed in US. Pat. No. 2,607,894, entitled Aerial System. However,conventional notch antenna design parameters require structures whichare unacceptably large for use in small aircraft. Therefore, where it isdesired to employ a structurally small antenna which can be flushmounted in virtually any type of aircraft, there has not, heretofore,been an entirely satisfactory answer. This is particularly so where itis desired to have an antenna which will function efficiently for bothtransmitting'and re-' ceiving.

SUMMARY OF THE INVENTION There is provided by the present invention atransmit/receive antenna comprising a physically short, folded dipolehaving its electrical length increased by means of end loading byferrite toroids or rings encircling the ends of the dipole radiator. Theentire assembly is flush-mounted in a notch made in the leading (ortrailing) edge of a conductive aerodynamic surface or the empennage ofthe aircraft. Both the dipole and the surface surrounding the notch areeffective to transmit or receive the radio-frequency (RF) energy. Theantenna is matched to a coaxial feed cable, over a broad frequency band,by means of tunable capacitor and inductance'elements integral with'theantenna. The tuning section is a parallel circuit which broadens thebandwidth of the antenna, and is tuned to the center frequency. One endof the antenna is grounded, thereby protecting it against lighteningStrikes. In a typical construction the *antennamay be operated in thefrequency range of 100 to 300 megahertz. The frequency dependance of theferrite used greatly increases the antennas effective bandwidth.

It is therefore an object of theinvention to provide a novel andefficienty flush-mounted transmit/receive antenna suitable for mountingin small mobile or airborne vehicles.

Another object of the invention is to provide a novel and improvedantenna of the flush-mounted type which is the functional equivalent ofa folded dipole and/or a notch antenna but of smaller physicaldimensions.

Still another object of the invention is to provide a novel and improvedantenna which is inherently immune to lightening strikes.

Yet another object of the invention is to provide a novel and improvedaircraft antenna suitable for both transmitting and receiving and whichis of unusually wide bandwidth and small physical size.

These and other objects of the invention will become better understoodupon consideration of the accompanying specification and drawings.

BRIEF DESCRIPTION OF THE DRAWINGS FIG.--l is a side elevation view,partially broken away, showing the details of construction of oneembodiment of the invention.

FIG. 2 is a perspective view illustrating an embodiment of the inventionsuitable for installation in the vertical stabilizer of an airplane.

DESCRIPTION OF THE PREFERRED EMBODIMENT connected to the centralconductor 7 of coaxial cableconnector 8, which is mounted in wall 2. Theouter (viz., shield) conductor of connector 8 is directly connected towall 2. A tunable inductance assembly, indicated generally at 9, isslidably mounted in wall 3 and extends outwardly therefrom so that itsouter terminus l 1 slidably engages the dipole element 5. This assembly(9) comprises a hollow conductive tube 12 having a pair of flattenedfinger portions 13 surrounding the dipole element 5 and which areclamped thereto by means of screw fastener 14. The other end of tube 12is mounted on, or otherwise secured to, an insulating flange 15 which inturn is carried on plate 16. Plate 17 is located on the opposite surfaceof wall 3. Plates 16 and 17 straddle an elongated slot 18 in wall 3,thereby permitting assembly 9 to be moved back and forth with respect toone end of element 5. Plates 17 and 18 are clamped to either side ofwall 3 by a pair of screw fasteners 21 and 22 or other suitable means.Rod 19 has an exterior threaded portion 24 which threadedly mates withan interior threaded portion of insulated flange 15. Rod 19 is coaxiallydisposed within and spaced apart from tube 12. Thumbwheel 25 is attachedto rod 19 for rotation therewith, and permits the rod to be moved in thedirection of arrow 26 by suitably rotating the thumbwheel 25. Rod 19,which is made of metal or other conductor, plus tube l2-comprises acoaxial tuning capacitor for incorporation into the antenna matchingnetwork to be described hereinafter. By sliding the entire assembly 9 indirection of arrow 27, the center frequency of the antenna may beestablished.

The tunable capacitor and inductance assembly (9) will match the antennato the input impedance of a 50 ohm transmission line over a broadfrequency band. Broad tuning is obtained by adjusting the location ofthecylindrical capacitor 9 along the dipole member 5 so as to match themid-band frequency. It is thereafter fixed in place with respect to thebase (e.g., wall 3). Fine tuning is then obtained by means of thecapacitor (12, 19) only. It will be appreciated by those versed in theart, that the location of capacitor 9 with respect to dipole member 5may be fixed as a design parameter and, therefore, need not be madeadjustable.

Dipole element 5 carries a plurality of ferrite rings at either end, theouter pair of which, in each instance, is provided with set screws toretain the rings in position on the tubular dipole element 5. Forexample, rings 31-34 are secured to element 5 by set screws 35 and 36.Similarly, the ten rings 37-46 are secured to the opposite end ofelement 5 by means of set screws 48 and 49. Each ring or toroid has anaxial bore slightly larger than the diameter of the dipole element. Asis well known to those versed in the art, and as is discussed in detailin the aforementioned U.S. Pat. No. 2,748,386, the length-to-diameterratio of the ferrite rings should be 8:1 or greater in order to takeadvantage of the heneflts of the high effective permeability.

Referring to FIG. 2 there is shown, by way of example, anotherembodiment of the invention installed in the vertical stabilizer 52 of asmall aircraft. As can be seen, a notch is cut into the leading edge 51of the stabilizer 52 and enclosing walls 53-55 of metal or otherconductive material define the notch. Walls 53 and 55 have been shapedto conform to the geometry of the stabilizer 52. A non-conductive ordielectric cover 56 encloses the notch and is faired into the leadingedge skin of the stabilizer 52 so that the aerodynamic properties of thevehicle remain unaltered as a result of installation of the antenna.Other elements shown in FIG. 2 are similar to like parts in theembodiment of FIG. 1. The notch defining structure (53-55) is securedto, and is electrically contiguous with, the outer conductive surface ofthe vehicle (52). Thus, the frame (53-55) is grounded to the vehicle.The antenna may be enclosed in a fiberglass fairing 56 conforming to theexterior contour of the vehicle skin. If desired, the notch portion ofthe antenna may be filled with dielectric foam once the desiredadjustments have been completed. The dipole (5) is preferably orientedin accordance with the desired polarization of the radiated energy.

Operation of the invention may be better understood from a considerationof generally similar antennas. Resonance of a simple notch antenna maybe obtained either by varying the physical dimensions of the notchalone, or by inserting a network which increases the excitation of thenotch area and makes the conducting surface into a radiating element. Inthe first case, the

notch is the radiating element; in the second case, most of theradiation emanates from the surrounding conductive surface with onlyminor radiation from the notch.

In the present invention, only a part of the conducting surfacefunctions as a radiating element while most of the energy radiates fromthe elements which are located within the notch area. The antennaresembles a short, folded dipole, but its electrical length is increasedby the addition of the ferrite loading rings at the input end, thegrounded end, or both. The dipole actually comprises a folded monopole(e.g., element 5) with a ground plane (e.g., wall 2) to provide thereflected image. A tunable inductance-capacitance (L-C) section, matchesthe antenna impedance to the impedance of the coaxial feed cable. Theportion of the dipole element (5) between the transmission line (8) andthe tuning assembly (9) comprises the matching section, and theremaining portion (viz., the portion between the tuning assembly 9 andthe opposite end (4)) comprises the excited section, of the antenna. The

notch antenna, but smaller in, size than a rectangular or annular slotantenna. The cutout size of the notch is a function of the type ferritematerial employed for rings 31-34 and 37-46 and, therefore, depend onthe effective permeability Ut The length of the dipole memberS andnumber of encircling ferrite rings (31-34 and 37-46) will bring thestructure into resonance with the desired frequency of operation. Thetuning assembly 9 comprises a parallel circuit which broadens thebandwidth and is tuned to the desired operating center frequency. Theferrite rings effectively shorten the electrical length of the dipoleelement 5 or the notch and act as reactive loading devices. Theseloading devices (31-34 and 37-46) change the current distribution alongthe antenna (1 and 5) and, therefore, produce a more efficient radiatorthan an unloaded radiator with the same physical dimensions. A high Qferrite should be selected to keep losses to a minimum.

In a practical implementation of the invention various types of ferritematerial may be used for the loading rings including those identified asmagnesium-iron TT105, nickeliron T112430 and Ferroxcube type IVD.Magnesium ferrite and nickel ferrite are similar in frequencysensitivity and therefore allow operation over the same band. Anickel-zinc type ferrite material, having the following characteristicsat 25 C, has been found to be especially suitable for use inthe'practice of the invention: 7

Initial permeability 40 at 1 MHZ Maximum permeability 1 15p SaturationFlux Density 2400 Gauss Residual Mangetism 750 Gauss Coercive Force 4.7Oersted Temperature Coefficient 0.1% max/C Between 25C to 50C CurriePoint 450C Lem Factor at 50 MHz 170 X 10'' Although there is a slightreduction in the power transfer efficiency resulting from the use offerrites, which efficiency reduction is attributable to magnetic losses,the advantages outweigh the losses. Specifically, the ferrite-loadedsemi-notch antenna of this invention is simpler in construction, has agreater bandwidth and is of considerably smaller size than either simplenotches or recessed dipole antennas. Furthermore, it allows fixed tuningand matching over practical bandwidths without expensive or complexmatching devices.

In a typical construction the antenna may comprise a inch diameteraluminum conduit, 15 inches in length, for the dipole radiator. Thedepth of the notch is approximately 7% inches. Two ferrite ringsencircle the dipole conductor near the feed point. Similarly, eightferrite rings are located at the opposite end of the antenna. Aconstruction of these dimensions will operate in the frequency rangefrom megahertz to 230 megahertz. Without any ferrite loading on theantenna, the minimum voltage standing wave ratio (VSWR), or resonance,occurs at 280 MHz. The addition of two ferrite rings at the input areaand eight ferrite rings at the grounded end of the antenna, shifts theresonance to MHz. The concurrent change in minimum VSWR will be fromapproximately 1.521 to very near 1:1. The ferrite loading tends tocreate a more uniform current distribution along the antenna element,and hence increases the effective height and radiation resistance for agiven length of the antenna. The improved RF current distribution makesa series-loaded antenna, constructed in accordance with the invention,superior to an unloaded antenna of equal physical dimensions.

From the foregoing it will be seen that the present invention providesan antenna meeting the objectives set forth hereinabove and which isreadily adapted to installation in small aircraft to replace simplenotch, slot, helix, or dipole antennas, used heretofore, therebyovercoming the limited effectiveness and/or the excessive drag of suchprior devices.

While there have been shown and described and pointed out thefundamental novel features of the invention as applied to preferredembodiments, it will be understood that various omissions andsubstitutions and changes in the form and details of the antennasillustrated may be madeby those versed in the art. For example, theorientation of the notch structure with respect to the airframe may bealtered in accordance with specified aerodynamic parameters andradiation patterns. Furthermore, the notch dimensions may be increasedor decreased in accordance with particular mechanical and electricaldesign requirements. Such modifications may be made without departingfrom the spirit of the invention; therefore, it is intended that theinvention be limited only as indicated by the scope of the followingclaims.

What is claimed is:

1. An antenna comprising:

means defining an elongated discontinuity in an electrically conductivesurface; an elongate conductive radiating element disposed within saiddiscontinuity in spaced relationship thereto and having its major axisextending along the longitudinal axis of said discontinuity; ferritemeans adapted to series load said conductive radiating element forincreasing the inductive reactance thereof; and

means for electrically interconnecting said discontinuity defining meansand said conductive element whereby said ferrite-loaded antenna elementfunctions in connection with said notch as the electrical equivalent ofa center fed folded dipole.

2. An antenna as defined in claim 1 including:

an inductive-capacitive tuning means connected between saiddiscontinuity defining means and said conductive radiating element fordetermining the operational center frequency of said antenna.

3. An antenna as defined in claim 1 wherein said discontinuity definingmeans comprises:

a generally U-shaped conductive frame defining a notch having its openend flush with said conductive surface.

4. An antenna as defined in claim 1 wherein said conductive radiatingelement comprises:

a tubular dipole conductor having first and second ends, said first endbeing driven and said second end being directly connected to saiddiscontinuity defining means.

5. An antenna as defined in claim 4 wherein said ferrite meanscomprises:

a ferrite ring coaxially disposed with respect to said driven end ofsaid dipole conductor. 7

6. An antenna as defined in claim 4 wherein said ferrite meanscomprises:

a ferrite ring coaxially disposed with respect to said second end ofsaid dipole conductor.

7. An antenna as defined in claim 4 wherein said ferrite meanscomprises:

a first ferrite ring coaxially disposed with respect to said driven endof said dipole conductor;

and a second ferrite ring coaxially disposed with respect to said secondend of said dipole conductor. 8. An antenna as defined in claim 1wherein said in- 1Q terconnecting means includes:

a two-conductor transmission line having one of its conductors connectedto said discontinuity defining means, and the other of its conductorsconnected to said elongate conductive radiating element.. 9. An antenna,comprising: a generally U-shaped conductive frame defining a notch; aconductive sheet secured to the periphery of said frame and extendingoutwardly therefrom for radiating and receiving radio-frequency signals;an elongate ferrite-loading antenna element disposed within said frameand having first and second ends, said first end comprising and inputend closely spaced from said notch and said second end being connectedto said frame; and a two-conductor transmission line having one of itsconductors connected to said input end of said antenna element, and theother of its conductors connected to said frame whereby saidferriteloaded antenna element functions in connection with said notch asthe electrical equivalent of a center fed folded dipole. 10. An antennaas defined in claim 9 including: an inductive-capacitance tuning meansconnected between said frame and said antenna element for determiningthe operational center frequency of said antenna. I '11. An antenna asdefined in claim 10 wherein said tuning means includes:

a coaxial capacitor having a fixed outer sleeve connected to saidantenna element, and an insulated inner cylinder electrically connectedto said frame. 12. An antenna adapted to be mounted in a cutout in theexternal surface of an aircraft, comprising:

a generally U-shaped conductive frame defining a notch mounted in saidcutout and having its open end flush with said exterior surface; aferrite loaded dipole element located within said frame; means groundingone end of said dipole element to one end of said frame;

a two-conductor transmission line having one of its conductors connectedto the other end of said dipole element and the other of its conductorsconnected to the other end of said frame whereby said ferrite-loadedantenna element functions in connection with said notch as theelectrical equivalent of a center fed folded dipole; and

a dielectric window covering said cutout.

13. An antenna as defined in claim 12 including:

a tuning capacitor means connected between said frame and said dipoleelement.

14. An antenna as defined in claim 13 including:

means for adjusting the capacitance of said tuning capacitor means.

1. An antenna comprising: means defining an elongated discontinuity inan electrically conductive surface; an elongate conductive radiatingelement disposed within said discontinuity in spaced relationshipthereto and having its major axis extending along the longitudinal axisof said discontinuity; ferrite means adapted to series load saidconductive radiating element for increasing the inductive reactancethereof; and means for electrically interconnecting said discontinuitydefining means and said conductive element whereby said ferrite-loadedantenna element functions in connection with said notch as theelectrical equivalent of a center fed folded dipole.
 2. An antenna asdefined in claim 1 including: an inductive-capacitive tuning meansconnected between said discontinuity defining means and said conductiveradiating element for determining the operational center frequency ofsaid antenna.
 3. An antenna as defined in claim 1 wherein saiddiscontinuity defining means comprises: a generally U-shaped conductiveframe defining a notch having its open end flush with said conductivesurface.
 4. An antenna as defined in claim 1 wherein said conductiveradiating element comprises: a tubular dipole conductor having first andsecond ends, said first end being driven and said second end beingdirectly connected to said discontinuity defining means.
 5. An antennaas defined in claim 4 wherein said ferrite means comprises: a ferritering coaxially disposed with respect to said driven end of said dipoleconductor.
 6. An antenna as defined in claim 4 wherein said ferritemeans comprises: a ferrite ring coaxially disposed with respect to saidsecond end of said dipole conductor.
 7. An antenna as defined in claim 4wherein said ferrite means comprises: a first ferrite ring coaxiallydisposed with respect to said driven end of said dipole conductor; and asecond ferrite ring coaxially disposed with respect to said second endof said dipole conductor.
 8. An antenna as defined in claim 1 whereinsaid interconnecting means includes: a two-conductor transmission linehaving one of its conductors connected to said discontinuity definingmeans, and the other of its conductors connected to said elongateconductive radiating element.
 9. An antenna, comprising: a generallyU-shaped conductive frame defining a notch; a conductive sheet securedto the periphery of said frame and extending outwardly therefrom forradiating and receiving radio-frequency signals; an elongateferrite-loading antenna element disposed within said frame and havingfirst and second ends, said first end comprising and input end closelyspaced from said notch and said second end being connected to saidframe; and a two-conductor transmission line having one of itsconductors connected to said input end of said antenna element, and theother of its conductors connected to said frame whereby saidferrite-loaded antenna element functions in connection with said notchas the electrical equivalent of a center fed folded dipole.
 10. Anantenna as defined in claim 9 including: an inductive-capacitance tuningmeans connected between said frame and said antenna element fordetermining the operational center frequency of said antenna.
 11. Anantenna as defined in claim 10 wherein said tuning means includes: acoaxial capacitor having a fixed outer sleeve connected to said antennaelement, and an insulated inner cylinder electrically connected to saidframe.
 12. An antenna adapted to be mounted in a cutout in the externalsurface of aN aircraft, comprising: a generally U-shaped conductiveframe defining a notch mounted in said cutout and having its open endflush with said exterior surface; a ferrite loaded dipole elementlocated within said frame; means grounding one end of said dipoleelement to one end of said frame; a two-conductor transmission linehaving one of its conductors connected to the other end of said dipoleelement and the other of its conductors connected to the other end ofsaid frame whereby said ferrite-loaded antenna element functions inconnection with said notch as the electrical equivalent of a center fedfolded dipole; and a dielectric window covering said cutout.
 13. Anantenna as defined in claim 12 including: a tuning capacitor meansconnected between said frame and said dipole element.
 14. An antenna asdefined in claim 13 including: means for adjusting the capacitance ofsaid tuning capacitor means.